Support fakes for testing

[cedvdb]

There are already some fakes out there:

• https://github.com/atn832/fake_cloud_firestore
• https://github.com/atn832/firebase_storage_mocks

I've forks of both with fixes

• https://github.com/cedvdb/fake_cloud_firestore
• https://github.com/cedvdb/firebase_storage_mocks

The issue with those libraries is that those are single maintainer, incomplete and sketchy. Having to fix those is time consuming. All props for the work that was done there by the maintainer however testing is an integral part of software development, it follows that having solid testing utilities maintained by the core team makes sense.